Kristallmarathon

The Kristallmarathon (Crystal Marathon) is an extreme underground running event held 500 meters (1,640 feet) below the surface inside a salt mine in Merkers, Germany.

Here are the key details about the race:
Location and Environment Venue: Merkers Adventure Mine (Erlebnis Bergwerk Merkers), Thuringia, Germany.

Conditions: Constant temperature of 21°C (70°F) with low humidity.

Gear: Wearing a bicycle helmet is strictly mandatory for all runners due to safety regulations.

Race Formats
Distances: Marathon (42.2 km), Half Marathon (21.1 km), and 10K.The
Course: A 3.25 km loop through the mine tunnels.
Laps: Marathoners complete 13 laps; half marathoners complete 7 laps.